Reaming the Engine Mount

[TASEsq]

I’m building a taildragger and I’m a bit confused about the reaming progress.

Firstly, the holes in the firewall have already been reamed to 3/8” back in chapter 27 when I attached the firewall.


This is the instructions in the taildragger page 40B-2:



So this leads me to think I have to ream the holes in the steel engine mount, but not “match ream” the mount to the airframe?

I ask, as I checked the wiki but it only had the nose wheel version of the plans, and that seems to indicate to ream both the mount and the firewall at the same time? (Which I’m not sure how you would do since you can’t really insert a bolt until it’s reamed). In the thread linked below, @rvbuilder2002 mentions that they are match drilled. How do to hold it in place if the bolts don’t go in?


Also - my holes are really not very well centered in the bushing of the firewall mount. I understand this is normal based on this thread, but wow - they are a fair way out.

Engine mount alignment to firewall

Has anyone ran into this issue? My upper right engine mount to firewall bolt hole is not aligned well with the engine mount. The result is that there is uneven pressure on the washer even after it is tightened. Is this a big problem? The bolt goes in, but is riding to one side of the hole of...

vansairforce.net

 

[Ron B.]

I’ve built both , the 14 and 14A. It’s been a while. I don’t think I would match ream as that could elongate the hole in the fuselage. The holes don’t line up perfectly and force is sometimes needed to make them line up. Actually installed three, last one did line up perfectly ( luck). I would ream the mount and fuselage separately. Then play with the installation of the bolts. Might have to try different locations to start inserting the bolts. You might want to use standard store bolts to start with and then trade them out once you get them to fit.

 

[rvbuilder2002]

How do to hold it in place if the bolts don’t go in?

You could use 3/16" diameter hardware store bolts with washers, to hold the mount in the position you want, and then exchange for the 3/8" bolts as you drill/ream each hole.

 

[TASEsq]

You could use 3/16" diameter hardware store bolts with washers, to hold the mount in the position you want, and then exchange for the 3/8" bolts as you drill/ream each hole.

Thanks for jumping in so fast Scott! Appreciate it.

I’m just having some trouble understanding the logic of this process.

The fuselage holes are already 3/8 and there’s no way to ream from the cockpit side forward into the engine mount.

If I were to use the undersized 3/16 bolts and ream from the forward side, the reamer will center in the existing hole in the engine mount. This means I would be reaming a hole that I can’t be sure is aligned with the hole below it. I’m sure on the first hole the reamer *might* pull the mount to the correct location and centre it. But on the last hole, with 3/8 bolts in all the other holes, I can’t imagine this is a good idea? Surely this will mean an elongated hole in the fuselage?

I’ve probably mis-understood what you are getting at.

 

[wirejock]

Vans used to sell a bushing kit for builders who retrofit the Elastomeric gear and mount to an existing 7A, 9A. Maybe it would work?

 

[TASEsq]

Vans used to sell a bushing kit for builders who retrofit the Elastomeric gear and mount to an existing 7A, 9A. Maybe it would work?

Thanks - the design is ok (the holes not being centered). It’s just an order of operations thing.

 

[rvbuilder2002]

Thanks for jumping in so fast Scott! Appreciate it.

I’m just having some trouble understanding the logic of this process.

The fuselage holes are already 3/8 and there’s no way to ream from the cockpit side forward into the engine mount.

If I were to use the undersized 3/16 bolts and ream from the forward side, the reamer will center in the existing hole in the engine mount. This means I would be reaming a hole that I can’t be sure is aligned with the hole below it. I’m sure on the first hole the reamer *might* pull the mount to the correct location and centre it. But on the last hole, with 3/8 bolts in all the other holes, I can’t imagine this is a good idea? Surely this will mean an elongated hole in the fuselage?

I’ve probably mis-understood what you are getting at.

Sorry, I was not involved in writing that section of the manual and it has been long enough now that I have forgotten some of the specifics of the process.
I was remembering that the engine mount was shipped with the attached holes, not being finished to final size, but apparently that is not the case.

I can tell you, that the design intent of the half inch diameter bosses at the engine mount attach points was to allow for the hole in the engine mount attach points to be drilled off center if necessary, to match up to the holes in the firewall.
This was done to try and avoid preloading the engine mount by pulling it into matching alignment, since every one of them tends to spring out of shape slightly to some degree when it is removed from the welding fixture.
This works out from an engineering standpoint because it is one of the few places for high structural loads that the fastener is predominantly loaded in tension. The special thick washer bridge is the load from the bolt into the larger diameter boss, and the sheer loads are taken where the engine mount contact the face of the firewall.

 

[TASEsq]

Sorry, I was not involved in writing that section of the manual and it has been long enough now that I have forgotten some of the specifics of the process.
I was remembering that the engine mount was shipped with the attached holes, not being finished to final size, but apparently that is not the case.

I can tell you, that the design intent of the half inch diameter bosses at the engine mount attach points was to allow for the hole in the engine mount attach points to be drilled off center if necessary, to match up to the holes in the firewall.
This was done to try and avoid preloading the engine mount by pulling it into matching alignment, since every one of them tends to spring out of shape slightly to some degree when it is removed from the welding fixture.
This works out from an engineering standpoint because it is one of the few places for high structural loads that the fastener is predominantly loaded in tension. The special thick washer bridge is the load from the bolt into the larger diameter boss, and the sheer loads are taken where the engine mount contact the face of the firewall.

No, you are correct. The engine mount ships with the holes undersized.

The firewall holes are reamed back in chapter 28, but the engine mount holes are not called to be reamed until chapter 40.

I can’t see a way to ream them except from aft to forward, but no really way to get access to do this.

It would have made more sense to ream the mount, then ‘match ream’ to the fuselage but too late now.